LIGHT REACTIONS AND PHOTOTAXIS 609 



turned on the yellow or the blue light. I concluded that there 

 is no sensitisation of the protoplasm of eye or body of the nauplii 

 by the dyes used and that their negativating influence is prob- 

 ably due to a non-photochemical effect on the protoplasm. 



Concluding from the information gained by my experiments, 

 on the behavior of free Balanus larvae under normal conditions, 

 it may be supposed that they react very similarly to the other 

 planktonic forms investigated by the present author. After 

 hatching, the larvae swim towards the surface, the strong in- 

 crease of light causing them to sink down again very soon by 

 inhibition of their locomotion. Their movements will consist 

 in a continuous alternation of sinking and rising ('periodical 

 locomotion,' Ewald '10) caused by successive inhibition and 

 stimulation, without ever necessitating the taking place of nega- 

 tive reaction. This reaction probably constitutes an artificial 

 product of the laboratory. The 'periodical locomotion' as de- 

 scribed in the paper referred to above, causes the animals to 

 maintain themselves in an area of equal illumination throughout 

 the day, taking them gradually up in the evening and down 

 in the morning. In the evening, decrease of illumination will 

 slowly shift the position where inhibition due to prolonged up- 

 ward locomotion takes place, nearer and nearer the surface, 

 while the reverse is the case for the morning. It is thus unnec- 

 essary to assume that the animals constantly change between 

 positive and negative reaction, as was supposed by Loeb. The 

 eiiiinent usefulness of this mechanism is shown by the experi- 

 ments demonstrating the strong deleterious effect of the light 

 rays of short wave lengths on the nauplii. 



SUMMARY 



/. Effects of light 



1. The nauplii of Balanus perforatus show the same reactions 

 to changes of intensity of locomotion, as that found by the 

 author in Cladocera and Copepods. Increase of illumination 

 causes inhibition of locomotion, preceded by a slight accelera- 

 tion; the result is a sinking. Decrease of illumination causes 



THE JOURNAL OP EXPERIMENTAL, ZOOLOGY, VOL 13, NO. 4 



